1. Field of the Invention
The present invention generally relates to all terrain vehicles. More particularly, present invention relates to recreational all terrain vehicles having power assisted steering arrangements useful to facilitate low speed steering and steering with a locked differential as well as to reduce high-speed steering kick-back caused by glancing blows to steerable wheels.
2. Description of the Related Art
All terrain vehicles currently are undergoing a transformation. Previously, the vehicles were rather simple in design, relatively low powered and featured few rider comforts associated with larger motor vehicles, such as automobiles. Recently, however, riders have been demanding more of their recreational vehicles and manufacturers have been accommodating the rider demands.
One recent modification to all terrain vehicles has been the increase of horsepower and the associated increase in speed. In the past few years, the displacement size of engines used in all terrain vehicles has increased dramatically. The increase in displacement has been made possible, at least in part, by advances in engine design and technology. These advances enable the output of the engine to be much higher, sometimes at the cost of increased vehicle weight. Other changes in vehicle design also have resulted in increased vehicle weight.
The increase in vehicle weight has several drawbacks that generally have been considered design tradeoffs for the advantages. One of these drawbacks is the high forces required to turn the steering wheels while the vehicle is stationary or moving very slowly. The steering system in many recreational all terrain vehicles generally comprises a steering handle assembly that is connected to a steering shaft. The lower end of the steering shaft carries a pitman arm and the pitman arm is connected to the steering wheels by connecting rods. While this simple system is easy to manufacture and maintain, the system becomes increasingly difficult to turn at low speeds as the vehicle weight increases.
In addition, when an all terrain vehicle features a four wheel drive transmission having a lockable differential, steering effort must be increased when the differential is locked. As is known, a differential generally allows a pair of driven wheels to be uncoupled such that one wheel can be driven at a faster speed than the other wheel. For instance, during cornering, one wheel will rotate faster than another wheel when the wheels are connected by a differential. If the differential is locked, however, both wheels attempt to turn at the same speed. Thus, the effort needed to steer the vehicle is increased because both steerable wheels are attempting to rotate at the same speed.
A further issue involving the steering systems of recreational all terrain vehicles is steering kickback. Due to the terrain over which recreational all terrain vehicles are often operated, the vehicles often have to negotiate rocks, logs and other relatively immovable barriers or obstacles. When one of the steerable wheels comes into contact with an obstacle, the wheel often is forced rearward at a rapid rate. The rearward movement of the wheel then is transmitted directly through the steering arrangement to the operator. The glancing blow to the wheel, thus, can cause kickback in the steering assembly and can cause a temporary change in vehicle direction. Thus, the kickback can increase the steering effort required from the operator.